This invention relates to a receiver capable of reproducing information signals with frame synchronization established.
As will later be described with reference to one figure of the accompanying drawing, a conventional receiver of the type described is supplied with an input signal comprising information signals and frame synchronization signals preceding the information signals, respectively. The information signals carry, for example, an audio signal in a digital form and are usually subjected to scrambling in a transmitter so as to avoid consecutive production of "0" or "1."
The receiver establishes frame synchronization with reference to the frame synchronization signals and carries out descrambling of the information signals to produce a reproduction of the information signals. Anyway, establishment of the frame synchronization resorts only to the frame synchronization signals.
As a rule, a plurality of frames are required in the receiver in order to establish frame synchronization from an asynchronous state. In other words, no reproduction is carried out in the receiver during the above-mentioned plurality of frames and results in interruption of the reproduction. In addition, the receiver is put into the asynchronous state when the frame synchronization signals are not detected due to variation of an error rate in a transmission path.
On the other hand, it has been found out that communication is often possible even when no frame synchronization signals are detected. If the reproduction was interrupted while no frame synchronizing signals are detected, an objectionable or unpleasing interruption of the communication often takes place despite the fact that the communication can be continued. Once the receiver is put into the asynchronous state, noise alone is reproduced during the objectionable interruption of the communication because the descrambling is carried out in a manner which is different from the scrambling of the transmitter.